1. Field of the Invention
This invention relates to a vapor concentrator that may interface to an instrument that detects trace quantities of chemicals present as vapors in air or other gases, or liberated as vapors from condensed phases, such as particles or solutions, and more particularly relates to the desorption of concentrated samples of such vapors for injection into analytical instruments.
2. Description of Related Art
The quantitative analysis and identification of trace organic vapor species is rapidly becoming important in a variety of applications. For example, trace vapors are used to indicate the presence or recent handling of various explosive compounds. Detection of toxic chemicals, narcotic substances, fumes from food decay, fumes from printer's inks used in the manufacture of money, medically significant compounds released by bacteria, and industrial processing chemical indicators are just some of the applications currently utilizing the analysis of trace chemical vapors. Often, the volume concentrations of these vapors in air will be in the range of parts per million (ppm) to parts per trillion (ppt) or even less.
A variety of highly sensitive gas analysis measurement techniques already exist for recognizing vapors in this concentration range, which include, but are not limited to, ion mobility spectroscopy (IMS), gas chromatography (GC), flash gas chromatography (fast GC), and mass spectroscopy (MS). The sensitivity of these well-known techniques is often limited by the low concentration of the target chemical vapors that are present at their input ports.
The volume concentration of vapor entering the input port of the analysis device can be increased compared to the ambient concentration using a device called a vapor concentrator or vapor preconcentrator. A variety of concentration designs have been described. See, for example, U.S. Pat. No. 4,987,767, which describes a rotating metal screen and heater device. Explosives vapors are adsorbed onto the metal screen in the concentration phase and subsequently desorbed by a heating phase at a different rotational position. See also U.S. Pat. No. 6,085,601, which describes an adsorbing filter screen substantially perpendicular to the air flow for use in the concentration phase, and resistively heating said screen in the desorbing phase. See also U.S. Pat. No. 6,239,428, which describes a concentrator consisting of either a permeable organic membrane or a thin metal foil that is heated for the desorbing phase. See also U.S. Pat. No. 5,585,375, which is similar to U.S. Pat. No. 4,987,767 but with a concentration-enhancing coating on the rotating metal screen, and U.S. Pat. No. 6,345,545, which is similar to U.S. Pat. No. 6,085,601 but with a multistage filter screen method.
All of these vapor concentration devices utilize the tendency of vapors to adsorb onto a surface. The surface may optionally be selectively coated with an adsorption-enhancing chemical. The adsorbing surface is subsequently heated to desorb the accumulated vapor, either by means of an included heating element or by means of an external oven. If the heating is external, thermal communication is by hot air, substantially continuous electromagnetic radiation, a heat pipe, or a mixture of these methods. An included heating element may be an electrically resistive heater, but may also be a semiconductor device using the Peltier effect. The resistive heater may be pulsed, and the temperature versus time profile may be controlled, to cause more volatile compounds to be preferentially desorbed at an earlier time in the cycle. High temperature surface cleaning steps may also be included in the total cycle of operation. Further improvements include multiple stages of independent concentration surfaces that may be sequentially heated to bunch the vapor into a short duration pulse or alternatively may be sequentially heated at different temperatures to induce separation of chemicals by volatility.
The vapor drawn from the concentration device may be a mixture of a carrier gas, such as ambient air, nitrogen, or an inert gas, the target molecule or molecules, and other possibly interfering chemicals. Separation, combined with identification of the target compound, is performed by the analytical instrument. The analytical instrument's sampling method may employ a gas pump, which draws the sample gas from the concentrator into the instrument through a tube. For example, the pump may be disposed to provide a partial vacuum at the exit of an ion source, which is a component of the analytical instrument. This partial vacuum may be transmitted through the confines of the ion source and appear at the entrance orifice of the ion source. A further tubulation may be provided as an extension to a more conveniently disposed sampling orifice external to the analytical instrument. Molecules of interest may undesirably adsorb onto surfaces in the sampling flow path that are not part of the concentration device. Therefore, heating of the connecting tubulation may be required to minimize loss of vapor.
All of these concentration methods are limited by the fact that the relatively large mass of the substrate must change temperature, a relatively slow process. The rate of temperature rise relates to the power used to heat the substrate of the adsorbing surface divided by the mass being heated divided by the heat capacity of the substrate. Thus, a low mass, as well as a low heat capacity, allows the adsorbing surface to be heated at a faster rate. This is particularly useful for portable devices where high power is not usually available from batteries.
The volume concentration of the concentrated vapors as presented to the analytical instrument depends on the release rate of target vapor off of the adsorbing surface relative to the flow rate of the carrier gas. If all of the adsorbed target vapor could be released more rapidly, the volume concentration of target vapor presented to the analytical instrument may be substantially increased, and the signal-to-noise ratio and sensitivity may be greatly improved.